In U.S. patent application Ser. No. 075,815 now U.S. Pat. No. 4,309,965 there is described a new wood fuel combustion system developed by Professor Richard C. Hill of the University of Maine at Orono and assigned to the Board of Trustees of the University of Maine. According to the system developed by Professor Hill and described in patent application Ser. No. 075,815, burning of wood fuel for heating purposes is accomplished first by combustion of wood-type fuel in a high temperature (e.g. 1200.degree.-2000.degree. F.) (650.degree. C.-1100.degree. C.) refractory environment; second by delayed propagation of the flue gases in a continuing high temperature refractory insulating environment to assure complete combustion; and third, only after completion of combustion, by extraction of heat from the end products of such combustion. Integrally combined with this system is an air tight chamber arrangement for vertically feeding wood-type fuel into the high temperature refractory environment while confining the locus of combustion to the base of the fuel only. This is achieved by a water jacket or water environment around the upper portion of the combustion chamber which quenches ignition of wood in the portion of the fuel over the locus of combustion. Combustion is therefore confined to the bottom of the fuel at the base of the chamber. The flue gas is laterally drafted away for a controlled steady state burn of the charge of fuel placed in the air tight chamber. The water jacket arrangement moreover is a coacting element of the water system cooperating with the heat exchanger and downstream elements for extracting heat. Yet the purpose of the water jacket arrangement is not only heat extraction but also confinement of combustion and controlled burning.
According to the Hill disclosure in Ser. No. 075,815, now U.S. Pat. No. 4,309,965, a substantially vertical feed combustion chamber forming an upright column receives and supports wood-type fuel in a substantially vertical attitude or stack. The combustion chamber is substantially air tight and formed with a laterally directed draft outlet at the base of the combustion chamber.
Hill also provides a flue gas delay propagation channel extending from the laterally directed draft outlet at the base of the combustion chamber, and a heat exchanger coupled to the output of the flue gas delay channel for receiving the hot gaseous end products of wood combustion and transferring heat from the gases to the water or other medium. The flue gas propagation channel affords delayed propagation of the flue gas and forced air mixture in a high temperature refractory environment sufficient to afford substantially complete secondary burning of the gaseous products and constituents of wood burning and all prior to heat transfer in the heat exchanger. The water jacket and heat exchanger form elements of a common water or heat transfer fluid environment.
Hill also provides an induced draft from the base of the combustion chamber through the delayed propagation channel and heat exchanger for laterally directing the draft away from the locus of wood fuel combustion and away from the vertically stacked charge of fuel. In addition to actively inducing the draft, the disclosed system actively forces air into the combustion chamber base portion to effect turbulent mixing and swirling of air with the gaseous products of wood fuel pyrolysis. A turbulent mixture of air and fuel gas therefore follows the draft from the base of the combustion chamber through the laterally directed channel where the travel time in a high temperature environment permits substantially complete secondary burning of the flue gases. Heat exchange following the chemical reaction phase thereafter affords low stack temperatures.
There is therefore, according to Hill, a cooperating water jacketed or water circulating environment at one level and high temperature refractory insulating environment at another level. There is controlled coupling between the levels with wood fuel passing from the higher heat exchanging water level environment to the lower refractory insulating level. There is passage of products of completed secondary combustion from the heat confining lower refractory environment back through the heat transferring water level environment. There is vertical feed of wood with confinement of combustion to the refractory level and there is delayed propagation of flue gas through the refractory level for complete combustion before return through the water level environment.
A limitation of the wood-type fuel burning system described by Professor Hill in Ser. No. 075,815 now U.S. Pat. No 4,309,965 is that the rate of fuel combustion is constant throughout the burn of a charge of fuel. The rate of fuel combustion may not be "turned down," and the furnace burns at capacity until the charge of wood is consumed. While this results in complete and efficient combustion, the rapidly produced high intensity heat energy must be conducted away as quickly and stored. Thus, the furnace can generally not be used directly as a space heater or in other direct applications requiring low impedance, low intensity heat energy.